Hot rolled steel having low compressive strength loss after being processed into steel pipe, and manufacturing method therefor

ABSTRACT

Provided are a hot rolled steel having small compressive strength loss after being processed into a steel pipe, and a manufacturing method therefor. The hot rolled steel of the present invention comprises, by wt %, 0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities, wherein 250&lt;450C+95Si+70Mn is satisfied, a microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8 to 25 μm, and a value defined by relation 1 is less than 20%.

TECHNICAL FIELD

The present disclosure relates to a hot rolled steel having lowcompressive strength loss after being processed into a steel pipe, and amanufacturing method therefor.

BACKGROUND ART

Recently, as a next-generation transportation system, research into ahigh-speed vacuum tube train known as a hyperloop has been activelyconducted both domestically and internationally. The high-speed vacuumtube train is basically a transportation means in the form of moving atrain within the vacuum tube. That is, since the high-speed vacuum tubetrain system minimizes air resistance by maintaining an interior of thetube in a vacuum state, it is a transportation means based on theconcept that the train may run at high speed.

According to recent trends, it is known that metal steel pipes are moreadvantageous than concrete in terms of maintaining a vacuum in a tube.In addition, in consideration of the strength and manufacture costs oftube materials, it is true that steel pipes are the most suitable.

As a basic method of manufacturing a steel pipe, if a hot rolledmaterial is required, a method of using a steel pipe prepared by weldinga material after the material is subject to a separate heat treatmentand a bending process in the form of a steel pipe is known.

On the other hand, in the case of metal materials, it is well known thatmaterials subjected to tensile stress once are affected by theBauschinger effect in which the yield strength of compressive stressdecreases. According to a steel pipe manufacturing process, a steelmaterial receives tensile stress in a circumferential direction of asteel pipe during a bending process, which may result in a compressivestrength loss in the same circumferential direction after processing.Compressive strength loss in the circumferential direction of the steelpipe not only may cause shape deformation after processing a material,but also cause serious defects that may lead to tube collapse inemergency situations. Therefore, in the steel pipe material for a tube,a steel material that can minimize compressive strength loss needs to beapplied.

RELATED ART DOCUMENT Patent Document

-   (Patent Document 1) International Publication No. WO2005080621A1

SUMMARY OF INVENTION Technical Problem

An aspect of the present disclosure is to provide a hot rolled steelhaving low compressive strength loss after being processed into a steelpipe, and a manufacturing method therefor.

The subject of the present disclosure is not limited to the above. Aperson skilled in the art would have no difficulty in understanding thefurther subject matter of the present disclosure from the generalcontent of this specification.

Solution to Problem

According to an aspect of the present disclosure,

-   -   a hot rolled steel having small compressive strength loss after        being processed into a steel pipe may include, by wt %, 0.15% or        less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less        of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N)        and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe        and inevitable impurities, wherein 250<450C+95Si+70Mn is        satisfied,    -   a microstructure is a mixed structure of ferrite and pearlite,        the average grain size of the ferrite is 8 to 25 μm, and    -   when tensile strength under a 0.3% stretching condition is        referred to as σ_(Tensile) and compressive strength when the        stretched and deformed material is compressed by 0.2% again is        referred to as σ_(compressive), a value is less than 20% defined        in the following Relational Expression 1.

$\begin{matrix}{\frac{( {\sigma_{Tensile} - \sigma_{compressive}} )}{( \sigma_{Tensile} )} \times 100} & \lbrack {{Relational}{Expression}1} \rbrack\end{matrix}$

The tensile strength σ_(Tensile) of the hot rolled steel may be 385 MPaor more, and impact toughness at 0° C. may be 50 J or more.

In addition, according to an aspect of the present disclosure,

A method for manufacturing a hot rolled steel having small compressivestrength loss after being processed into a steel pipe may include:finish hot-rolling a steel slab including the above-describedcomposition at a finish temperature of 950 to 1030° C.; and

-   -   cooling the finish hot-rolled steel sheet to a temperature        within a range of 580 to 730° C. at a cooling rate of 5 to        50° C. and coiling the finish hot-rolled steel sheet,    -   wherein a temperature change of the cooled hot-rolled steel        sheet is controlled to be within 20° C. for 3 seconds        immediately before coiling it.

Advantageous Effects of Invention

According to an aspect of the present disclosure, there may be provideda steel having small compressive strength loss after being processedinto a steel pipe, in which when tensile strength under a 0.3%stretching condition is referred to as σ_(Tensile) and compressivestrength when the stretched and deformed material is compressed by 0.2%again is referred to as σ_(compressive) a value is less than 20% definedin the above-described Relational Expression 1.

BEST MODE FOR INVENTION

Hereinafter, exemplary embodiments in the present disclosure will bedescribed.

The present disclosure relates to technology for manufacturing a steelhaving low compressive strength loss during manufacturing of a steelpipe, and specifically, the steel including: by wt %, 0.15% or less ofcarbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese(Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in asum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities,wherein 250<450C+95Si+70Mn is satisfied, a microstructure is a mixedstructure of ferrite and pearlite, an average grain size of the ferriteis 8 to 25 μm, and when tensile strength under a 0.3% stretchingcondition is referred to as σ_(Tensile) and compressive strength whenthe stretched and deformed material is compressed by 0.2% again isreferred to as σ_(compressive), a value is less than 20% defined in theRelational Expression 1.

[Steel Composition]

First of all, steel composition components of steel for manufacturing asteel pipe of the present disclosure and reasons for limitation thereofwill be described. Hereinafter, “%” refers to “% by weight” unlessotherwise specified.

Carbon (C): 0.15% or Less

Carbon (C) is a representative hardenability improving element and is anelement that effectively contributes to securing the strength of steel.Accordingly, the present disclosure may include carbon (C) in a range ormore satisfying 250<450C+95Si+70Mn from a viewpoint of securing thestrength of a vacuum tube structure. On the other hand, when the contentof carbon (C) is excessively added, toughness of the steel may decreaseand weldability may decrease. Accordingly, the present disclosure maylimit an upper limit of the carbon (C) content to 0.15%. Morepreferably, the upper limit of the content of carbon (C) is limited to0.12%.

Silicon (Si): 2.5% or Less

Silicon (Si) is an element contributing to the deoxidation of steel.Accordingly, the present disclosure may include silicon (Si) of250<450C+95Si+70Mn or more to secure the cleanness and strength ofsteel. On the other hand, when silicon (Si) is excessively added,high-temperature strength of a material may be increased to cause aproblem in a continuous casting process, and the surface quality of aproduct may be deteriorated by preventing separation of surface scale.Accordingly, the present disclosure may limit an upper limit of thecontent of silicon (Si) to 2.5%. More preferably, the upper limit of thecontent of silicon (Si) is limited to 2.0%.

Manganese (Mn): 2.0% or Less

Since manganese (Mn) is an element contributing to improving thehardenability of steel, the present disclosure may include manganese(Mn) of 250<450C+95Si+70Mn or more to secure the strength of steel. Onthe other hand, when manganese (Mn) is excessively added, toughness ofthe steel may be deteriorated and weldability may be reduced.Accordingly, the present disclosure may limit an upper limit of thecontent of manganese (Mn) to 2.0%. More preferably, the upper limit ofthe manganese (Mn) content is limited to 1.8%.

Aluminum (Al): 0.05% or Less

Aluminum (Al) is an element that easily reacts with oxygen, and is arepresentative element used in a deoxidation reaction during asteelmaking process. However, when aluminum (Al) is present in steel,since there is a concern of generating an inclusion, it is preferable tocontrol aluminum (Al) so that aluminum (Al) does not remain in the steelas much as possible. Accordingly, the present disclosure may limit anupper limit of the content of aluminum (Al) to 0.05%.

Contents of Nitrogen (N) and Boron (B) in a Sum of: 0.002% to 0.008%

Nitrogen (N) and boron (B) are interstitial solid solution elements, andalthough their content in steel is lower than that of other elements,they have a relatively large effect on physical properties. The presentinventors have found that the sum of the two element contents is relatedto a compressive strength loss of the material. That is, when the sum ofthe two elements exceeds 0.008%, by wt %, it was confirmed that it isconsiderably difficult to control a value defined in RelationalExpression 2 described below to be less than 20. Furthermore, it is notpreferable to control the sum of the two elements to be less than 0.002%due to high possibility for a cost increase in terms of materialcomponent control.

Accordingly, in order to control the value defined in RelationalExpression 2 described below to be less than 20 in an economical manner,the sum of the contents of the two elements may be limited to 0.002 to0.008%, by wt %. More preferably, the sum of the contents of the twoelements is limited to 0.003 to 0.007%, by wt %.

250<450C+95Si+70Mn

In the present disclosure, it is required to control the contents ofcarbon (C), manganese (Mn), and silicon (Si) so as to satisfy theaforementioned inequation. If a calculation value of 450C+95Si+70Mn is250 or less, material strength may be deteriorated.

[Steel Microstructure]

The steel of the present disclosure consists of a mixed structure offerrite and pearlite.

In the present disclosure, a fraction of the ferrite may be 60 to 90%,in area %, and a fraction of the pearlite may be 10 to 40%, in area %.

The present inventors have repeatedly studied a method for reducing acompressive strength loss after stretching a steel material, and as aresult, they found that it is advantageous to reduce compressivestrength loss when the size of ferrite particles in the steel materialis large. However, when an average grain size of the ferrite is 25 μm ormore, since the impact toughness is too low for the ferrite to beapplied as a structure material, an upper limit of the average grainsize is limited to 25 μm. Furthermore, in order to realize a targetcompressive strength loss reduction in the present disclosure, since itwas confirmed that the average crystal grain diameter needs to be 8 μmor more, a lower limit was set to 8 μm.

[Property of Steel]

When tensile strength under a 0.3% stretching condition is referred toas σ_(Tensile) and compressive strength when the stretched and deformedmaterial is compressed by 0.2% again is referred to as σ_(compressive)the steel of the present disclosure may represent a compressive strengthloss of less than 20% defined in the following Relational Expression 1.

$\begin{matrix}{\frac{( {\sigma_{Tensile} - \sigma_{compressive}} )}{( \sigma_{Tensile} )} \times 100} & \lbrack {{Relational}{Expression}1} \rbrack\end{matrix}$

The present inventors simulated a situation in which a hot-rolled steelis subjected to tensile stress during steel pipe processing and asituation in which the corresponding steel pipe is subjected tocompressive stress in an actual use environment, and as a result of thesimulation, they found that the tensile stress is 0.3% and thecompressive deformation is 0.2%, respectively. In addition, in order toavoid the risk of shape deformation and rapid collapse of the steel pipeduring processing, they confirmed that it is necessary to control thevalue defined in the Relational Expression 1 to be less than 20%.

Meanwhile, in order for a steel pipe-processed material of the presentdisclosure to be used as a structural material, it is important tosecure a certain level of strength and toughness. Accordingly, the steelof the present disclosure may have a tensile strength σ_(Tensile) of 385MPa or more and an impact toughness at 0° C. of 50 J or more.

[Method for Manufacturing Steel]

Next, a method of manufacturing a hot rolled steel with little loss ofcompressive strength after processing a steel pipe of the presentdisclosure will be described.

The method for manufacturing steel according to the present disclosureincludes: finish hot-rolling a steel slab including the above-describedcomposition at a finish temperature of 950 to 1030° C.; and cooling thefinish hot-rolled steel sheet to a temperature within a range of 580 to730° C. at a cooling rate of 5 to 50° C. and coiling the finishhot-rolled steel sheet, and a temperature change of the steel sheet iscontrolled to be within 20° C. for 3 seconds immediately before coilingthe cooled hot-rolled steel sheet.

First, in the present disclosure, the steel slab having theabove-described composition is finish hot-rolled at a finish temperatureof 950 to 1030° C.

The finish hot-rolling temperature is an operational factor that has alarge effect on the austenite grain size (AGS) of the material. Ingeneral, the AGS is known to have a very high correlation with theferrite grain size (FGS), which is a cooling structure. In other words,when the AGS is coarse, the FGS is coarse and vice versa. The presentinventors of the present disclosure found that in order to control theFGS to be 8 to 25 μm, the finish hot-roll pressing temperature should be950° C. or more and 1030° C. or less. When the finish hot-rollingtemperature is less than 950° C., the AGS becomes very fine and the FGSalso comes out to be less than 8 μm. Conversely, when the finishhot-rolling temperature exceeds 1030° C., the FGS may exceed 25 μm,which may be disadvantageous in terms of impact toughness.

Then, in the present disclosure, the finish hot-rolled steel sheet iscooled to a temperature within a range of 580 to 730° C. at a coolingrate of 5 to 50° C. and then coiled.

The cooling rate when cooling the finish hot-rolled steel sheet affectsthe FGS, a surface scale, and material deviation inside the steel sheet.When the cooling rate is less than 5° C./s, the FGS becomes more than 25μm, and the surface scale is thick, which may result in a decrease in asteel sheet recovery. On the other hand, when the cooling rate exceeds50° C./s, there may be a problem in that the FGS may become finer toless than 8 μm.

In addition, a cooling end temperature during the cooling is relevant tohigh temperature strength for coiling a coil and the FGS for steel. Whenthe cooling end temperature is less than 580° C., the strength of thesteel during the coiling is high, which may result in a facility loadproblem during the coiling, and may cause the FGS to be less than 8 μm.On the other hand, when the cooling end temperature exceeds 730° C., theFGS exceeds 25 μm, which may cause a compressive strength loss valueaccording to the impact toughness and Relational Expression 1 describedabove to be 20% or more. Accordingly, in the present disclosure, thehot-rolled steel sheet needs to be cooled at a cooling end temperatureof 580 to 730° C.

Meanwhile, the surface temperature of the cooled steel sheet isincreased by heat recuperation, and the present inventors have foundthat when the steel sheet is coiled during the heat recuperationprocess, this affects the final FGS. Although an exact principle thereofis unknown, the inside and outside of the steel sheet reach thermalequilibrium during the heat recuperative process, and in this case, whenstress is applied during the coiling, it is determined that the FGS isaffected by factors such as dislocation. In consideration of this, inthe present disclosure, the temperature change of the steel sheet for 3seconds immediately before the coiling is limited to less than 20° C.This is because the FGS may exceed 25 μm if the temperature change ofthe steel sheet exceeds 20° C. for 3 seconds immediately before thecoiling.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail throughembodiments.

Example

After heating each steel slab provided with alloy compositions of Table1 at a temperature within a range of 1250° C., a hot rolled steel sheethaving the thickness of 10 mm was produced by controlling the finishhot-rolling temperature, the cooling rate after the hot rolling, thecooling end temperature, and the temperature change of the steel sheetfor 3 seconds immediately before the coiling. In addition, for each ofthe manufactured steel sheets, the FGS was measured in themicrostructure thereof and shown in Table 3 below, and compressivestrength loss value was measured to show results thereof in Table 3below. In addition, the impact toughness at 0° C. for each of themanufactured hot rolled steel sheet was measured, and results thereofwere also shown in Table 3.

Meanwhile, in Table 3 below, the FGS was measured using an opticalmicroscope with 500× magnification after etching each specimen by aNital etching manner, but tensile and compression tests were basicallyconducted in accordance with ASTM Standard E606-04, and, σ_(Tensile) andσ_(compressive) measured compressive strength loss using the valuedefined in the present disclosure. In addition, the impact toughness wasmeasured by a V-notch impact test method of KS B 0810. Meanwhile, inTables 2 and 3 below, Inventive Examples 1 to 9 had a mixed structure offerrite and pearlite, and Comparative Examples 1 to 14 also had a mixedstructure of ferrite and pearlite.

TABLE 1 Steel Sheet Steel Composition Element (wt %) No. C Si Mn Al N BN + B A*  1 0.06 1.8 0.8 0.04 0.0027 0.0005 0.0032 254.0  2 0.12 1.2 1.30.02 0.0061 0.0002 0.0063 259.0  3 0.1  1.5 1.1 0.03 0.0014 0.00080.0022 264.5  4 0.05 2.0 1.1 0.01 0.0068 0.0003 0.0071 289.5  5 0.15 1.61.2 0.05 0.0077 0.0003 0.0080 303.5  6 0.08 2.2 1.2 0.03 0.0016 0.00040.0020 329.0  7 0.08 2.5 1.0 0.02 0.0041 0.0012 0.0053 343.5  8 0.1  2.01.6 0.02 0.0027 0.0006 0.0033 347.0  9 0.13 1.8 2.0 0.03 0.0021 0.00070.0028 369.5 10 0.05 1.5 1.2 0.04 0.0037 0.0011 0.0048 249.0 11 0.08 1.41.1 0.05 0.0002 0.006  0.0062 246.0 12 0.08 1.5 1.0 0.01 0.0034 0.00030.0037 248.5 13 0.12 1.3 1.3 0.02 0.0070 0.0011 0.0081 268.5 14 0.08 2.21.1 0.03 0.0076 0.0006 0.0082 322.0 *In Table 1, A* is 450 C + 95 Si +70 Mn.

TABLE 2 Condition of manufacturing steel sheet Temperature Change ofSteel Sheet for 3 Seconds Finish Hot Cooling Immediately Steel RollingCooling End before Sheet Temperature Speed Temperature Coiling No.Division (° C.) (° C./s) (° C.) (° C.) 1 IE 1 992 29 690 15 2 IE 2 103012 620 20 3 IE 3 987 34 664 3 4 IE 4 950 18 730 8 5 IE 5 966 17 703 12 6IE 6 1022 5 580 7 7 IE 7 1013 22 616 6 8 IE 8 975 44 686 12 9 IE 9 99850 643 10 10 CE 1 967 25 585 15 11 CE 2 1015 20 650 14 12 CE 3 1028 16725 18 13 CE 4 1030 33 620 26 14 CE 5 1022 22 580 27 4 CE 6 949 16 72810 5 CE 7 1031 15 701 6 4 CE 8 951 4 726 16 5 CE 9 963 51 710 13 4 CE 10948 18 579 8 5 CE 11 966 17 731 12 1 CE 12 995 25 687 21 2 CE 13 1022 16615 22 *In Table 2, IE: Inventive Example, CE: Comparative Example.

TABLE 3 Steel Impact Sheet FGS σ_(Tensile) σ_(compressive) Toughness No.Division (μm) (MPa) (MPa) B* (% ) (J)  1 IE 1 21 386 320 17 52  2 IE 218 387 317 18 53  3 IE 3 15 392 314 20 54  4 IE 4  8 431 362 16 60  5 IE5 25 385 339 12 50  6 IE 6 10 433 368 15 57  7 IE 7 13 452 371 18 54  8IE 8 16 453 390 14 53  9 IE 9 19 462 388 16 52 10 CE 1 15 384 319 17 5411 CE 2 17 382 313 18 53 12 CE 3 20 383 314 18 52 13 CE 4  7 387 306 2164 14 CE 5  6 433 338 22 65  4 CE 6  7 431 340 21 62  5 CE 7 26 385 33912 49  4 CE 8 27 431 362 16 48  5 CE 9  7 385 300 22 56  4 CE 10 6 431340 21 65  5 CE 11 26 385 339 12 48  1 CE 12 26 386 324 16 48  2 CE 1326 387 329 15 47 *In Table 3, B* represents compressive strength lossvalue (%) defined in Relational Expression 1. In Table 3, IE: InventiveExample, CE: Comparative Example.

As shown in Tables 1 to 3, in the case of Inventive Examples 1 to 9 inwhich not only the steel composition components but also the steelmanufacturing conditions satisfy the scope of the present disclosure, itcan be seen that the FGS is within a range of 8 to 25 μm and compressivestrength loss value defined in Relational Expression 1 was less than20%, which was excellent. In addition, it can be seen that as the impacttoughness value at 0° C. was 50 (J) or more, a toughness value wasexcellent.

On the other hand, in Comparative Examples 1 to 3, under the steelmanufacturing conditions, the tensile strength σ_(Tensile) of the hotrolled steel sheet manufactured at a value of 450C+95Si+70Mn of 250 orless in the steel composition components within the scope of the presentdisclosure was not good below 385 MPa.

In Comparative Examples 4 and 5, the steel manufacturing conditions werewithin the scope of the present disclosure, but the N+B content in thesteel composition components deviated from the scope of the presentdisclosure, and from this, it can be seen that the FGS of themanufactured hot rolled steel sheet was less than 8 μm and thecompressive stress loss value was more than 20%, which were not good.

In addition, in Comparative Example 6, the steel composition componentwas within the scope of the present disclosure, but the finishhot-rolling temperature was considerably low, from which it can be seenthat the FGS of the manufactured hot rolled steel sheet was less than 8μm and the compressive stress loss value was more than 20%, which werenot good. In addition, Comparative Example 7 was a case in which thefinish hot-rolling temperature was too high, and the FGS of themanufactured hot rolled steel sheet exceeded 25 μm and the impacttoughness value was less than 50 (J), which were not good.

In addition, Comparative Example 8 was a case in which the steelcomposition component was within the scope of the present disclosure,but the cooling rate was too small in the steel manufacturing condition,and the FGS of the manufactured hot rolled steel sheet exceeded 25 μmand the impact toughness value was less than 50 (J), which were notgood. In addition, in Comparative Example 9, as the cooling rate was toohigh, it can be seen that the FGS of the manufactured hot rolled steelsheet was less than 8 μm and the compressive stress loss value was 20%or more, which were not good.

In addition, Comparative Example 10 was a case in which the steelcomposition component was within the scope of the present disclosure,but the cooling end temperature was too low in, and the FGS of themanufactured hot rolled steel sheet was less than 8 μm and thecompressive stress loss value was more than 20%, which were not good. Inaddition, in Comparative Example 11, as the cooling end temperature wastoo high, the FGS of the manufactured hot rolled steel sheet exceeded 25μm and the impact toughness value was less than 50 (J), which were notgood.

Furthermore, Comparative Examples 12 and 13 were a case in which thesteel composition component was within the scope of the presentdisclosure, but the temperature change of the steel sheet for 3 secondsimmediately before the coiling exceeded 20% in the steel manufacturingcondition, and the FGS of the manufactured hot rolled steel sheetexceeded 25 μm and the impact toughness value was less than 50 (J),which were not good.

As described above, preferred embodiments of the present disclosure havebeen described in the detailed description, but various changes andmodifications may be suggested to one skilled in the art withoutdeparting from the scope of the present disclosure. Therefore, the scopeof present disclosure should not be limited to the described embodimentsand should be determined not only by the claims described below but alsoby equivalents thereof.

1. A hot rolled steel having small compressive strength loss after beingprocessed into a steel pipe, the hot rolled steel comprising: by wt %,0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or lessof manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) andboron (B) in a sum of 0.002 to 0.008%, and a balance of Fe andinevitable impurities, wherein 250<450C+95Si+70Mn is satisfied, amicrostructure is a mixed structure of ferrite and pearlite, an averagegrain size of the ferrite is 8 to 25 μm, and when tensile strength undera 0.3% stretching condition is referred to as σ_(Tensile) andcompressive strength when the stretched and deformed material iscompressed by 0.2% again is referred to as σ_(compressive), a value isless than 20% defined in the following Relational Expression 1.$\begin{matrix}{\frac{( {\sigma_{Tensile} - \sigma_{compressive}} )}{( \sigma_{Tensile} )} \times 100} & \lbrack {{Relational}{Expression}1} \rbrack\end{matrix}$
 2. The hot rolled steel having small compressive strengthloss after being processed into a steel pipe of claim 1, wherein thetensile strength σ_(Tensile) of the hot rolled steel is 385 MPa or more,and impact toughness at 0° C. is 50 J or more.
 3. A method formanufacturing a hot rolled steel having small compressive strength lossafter being processed into a steel pipe, the method comprising: finishhot-rolling a steel slab including, by wt %, 0.15% or less of carbon(C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05%or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002to 0.008%, and a balance of Fe and inevitable impurities at a finishtemperature of 950 to 1030° C.; and cooling the finish hot-rolled steelsheet to a temperature within a range of 580 to 730° C. at a coolingrate of 5 to 50° C. and coiling the finish hot-rolled steel sheet,wherein a temperature change of the cooled hot-rolled steel sheet iscontrolled to be within 20° C. for 3 seconds immediately before coilingit.
 4. The method for manufacturing a hot rolled steel having a smallcompressive strength loss after being processed into a steel pipe ofclaim 3, wherein in the coiled hot rolled steel, a microstructure is amixed structure of ferrite and pearlite, and an average grain size ofthe ferrite is 8 to 25 μm, and when tensile strength under a 0.3%stretching condition is referred to as σ_(Tensile) and compressivestrength when the stretched and deformed material is compressed by 0.2%again is referred to as σ_(compressive), a value is less than 20%defined in the following Relational Expression
 1. $\begin{matrix}{\frac{( {\sigma_{Tensile} - \sigma_{compressive}} )}{( \sigma_{Tensile} )} \times 100} & \lbrack {{Relational}{Expression}1} \rbrack\end{matrix}$
 5. The method for manufacturing a hot rolled steel havinga small compressive strength loss after being processed into a steelpipe of claim 4, wherein the tensile strength (Tensile of the hot rolledsteel is 385 MPa or more, and impact toughness at 0° C. is 50 J or more.